This invention relates to a vapor generator and more particularly to a vapor generator which incorporates a fluidized bed furnace portion that has been retrofitted in place of a conventional burner-type design.
The use of fluidized beds has long been recognized as an attractive way of generating heat. In a fluidized bed arrangement air is passed through a perforated plate, or grid, supporting particulate fuel material. As a result of the air passing through the bed, the bed behaves like a boiling liquid which promotes the combustion of fuel. The basic advantages of such an arrangement include a relatively high heat transfer rate, a substantially uniform bed temperature, combustion at a relatively low temperature, ease of handling the coal, a reduction in corrosion and boiler fouling, and a reduction in boiler size.
Fluidized beds have enjoyed increased popularity especially with the advent of stringent pollution control requirements, since a material can be placed in the fluidized bed which absorbs the sulfur generated as a result of the combustion of the particulate fuel, resulting in a substantial reduction in air pollution.
These type of fluidized beds lend themselves to use in a vapor generator in which the heat from the bed is used to heat water flowing through heat exchange tubes, some of which form the walls of the generator. However, it is very difficult to retrofit an existing vapor generator incorporating a burner-type furnace with a fluidized bed furnace. One of the main reasons for this is that the vapor generator is usually top supported, i.e., the vapor generator hangs from a support system connected to its upper portion, while the fluidized bed is too heavy to be supported in such a manner and thus must rest on the floor. However, this presents the problem of joining the top of the fluidized bed module, which is relatively stationary, to the bottom of the boiler which thermally expands down and out. Thus a seal is required, but any type of metallic multifold type seal, or the like, would result in an extremely expensive expansion joint seal which, do to the stresses imposed by the differences in thermal expansion, would be susceptible to cracking and thus create potential maintenance problems.